The performance and life of ferrofluid seals is strongly influenced by the operating temperature of the ferrofluid.
"A temperature gradient is produced across the ferrofluid O-ring seal, as a result of the heat generated by the viscous shearing of the ferrofluid between the rotating spindle shaft and the inner diameter of the stationary pole pieces. The life of a ferrofluid rotary-seal apparatus may be extended through the proper selection of the seal housing material and housing geometry, so as to conduct heat away from the ferrofluid in one or more of the gaps of the seal. The rapid removal of heat from the ferrofluid permits a lower ferrofluid temperature during shaft operation, resulting in reduced ferrofluid loss and an extention of seal life. conducted away through the pole pieces and the spindle shaft. Thus, the operating ferrofluid temperature depends on the heat-sink capabilities of the seal materials and structure, which, in turn, determines the ferrofluid evaporation rate and, therefore, the life of the seal."
In general, mechanisms for removing heat generated in rotating ferrofluid seals are known. Examples of such mechanisms are described in U.S. Pat. No. 4,357,021, Raj in U.S. Pat. No. 4,357,022 issued to Raj et. al., Nov. 2, 1982, U.S. Pat. No. 4,340,233 issued July 22, 1982 and U.S. Pat. No. 4,357,023 issued to Yamamura on Nov. 2, 1982 describe means whereby heat generated in rotating ferrofluid seals may be removed. The various schemes described in the above cited patents are summerized in U.S. Pat. No. 4,357,023, column 2, line 57 through column 3, line 42.
However, in order for heat to reach the heat conductive housing, it must travel through magnetic pole pieces. In general, magnetic materials have poor thermal conductivity which, when combined with a relatively long thermal path, results in a large temperature differential between the ferrofluid seal and the heat conductive housing. Since ferrofluids should generally be operated at 50.degree. C. or less, a large temperature differential severely inhibits performance.
In this regard, the use of heat conductive extension elements disposed alongside the pole pieces have also been proposed. This approach however, is typically suitable only for one or two stage seals, i.e., external surfaces. If attempted for use in multi-stage seals, i.e., for vacuum use, the high thermal conductivity, nonmagnetic metal extension elements would interrupt the magnetic circuit, thereby compromising the seals effectiveness. Also, an undesirable temperature gradient is set up in radial direction.
Liquid cooling of a ferrofluid seal has been suggested in an article by Raj titled "Testing Magnetic Fluid Seals" in the March 1979 issue of Industrial Research/Development. Magnetic pole pieces are shown with channels for the flow of liquid coolant. However, in such design poor thermal conductivity of magnetic materials combined with relatively long thermal paths result in large temperature differentials between ferrofluid seal and the coolant. Also, this type of design generally results in a bulky structure.